The invention pertains to a method of operating gas turbines, as for example in conventional gas turbine power plants or a compressed air energy storage (CAES) system, and in particular to a method of operating gas turbines having multiple burners.
The layout of a conventional gas turbine power plant with a compressor is generally known.
The layout of an example of a compressed air energy storage (CAES) system is shown schematically in FIG. 1. It comprises a cavern 1 for the storage of compressed air used for in particular for the generation of power during high demand periods. The compressed air is admitted by the control of a valve arrangement 9 to a recuperator 2 where it is heated by heat transfer from exhaust from the gas turbine. A further valve arrangement 8 controls the admission of the compressed air to an air turbine 3. A combustion chamber 4 and gas turbine 5 are arranged downstream of the air turbine 3. An auxiliary burner 6 is arranged following the gas turbine 5 and before the recuperator 2. In this CAES arrangement the gas turbine combustion chamber 4 comprises multiple burners. When the gas turbine 5 is operated at various combustion chamber heat loads the number of active burners is varied. An automatic activation or deactivation of individual burners or burner groups by means of a gas turbine controller requires one criterion or several criteria that define the switch points for the burners. For example, this criterion can be the gas turbine load.
For premixed combustion taking place in a gas turbine combustion chamber the combustion event can be characterized by the equivalence ratio "PHgr" given by the ratio of the mass flow rate of fuel to that of oxidizer where the oxidizer usually is air. This is expressed by equation 1:   Φ  =      afr    ⁢                            m          .                fuel                              m          .                ox            
The factor afr (abbreviation for air fraction, generally the oxidizer being air) is the ratio of oxidizer mass flow to fuel mass flow for stoichiometric combustion, i.e. for a complete chemical reaction, where neither oxidizer nor fuel residuals are present in the exhaust gas.
By definition the equivalence ratio "PHgr" can take on any value between zero and infinity. However, for technical combustion the range is given by the flame stability limits. These limits are approached when the reaction cannot release enough heat to sustain chemical reaction and the flame subsequently extinguishes. This can happen if either excessive oxidizer or fuel is present. In the first case, the stability limit is defined as the xe2x80x9clean extinction limitxe2x80x9d.
The combustion emissions, in particular NOx, correlate strongly with the flame temperature. Flame temperature and hence emissions can be controlled by varying the amounts and distribution of fuel and oxidizer in the combustion chamber.
For a gas turbine having multiple burners, a single burner equivalence ratio "PHgr"SB is given by equation 2:       Φ    SB    =      afr    ⁢                                        m            .                                fuel            ,            SB                                                m            .                                air            ,            SB                              .      
It is an important indicator of flame temperature, combustion stability and emissions. Furthermore, a combustion chamber equivalence ratio is defined by equation 3:       Φ    CC    =      afr    ⁢                            m          .                          fuel          ,          CC                                      m          .                          air          ,          CC                    
The value of "PHgr"CC, together with the combustion chamber air inlet temperature and the fuel temperature, determines the firing temperature of the combustion chamber.
In a similar manner, the gas turbine equivalence ratio "PHgr"GT is related to the gas turbine inlet mix temperature TGT TIT, which is an important parameter of the overall gas turbine operation. This equivalence ratio is given by equation 4:       Φ    GT    =      afr    ⁢                                        m            .                                fuel            ,            GT                                                m            .                                air            ,            GT                              .      
The gas turbine air mass flow, the combustion chamber air mass flow and the air admission to one single burner are determined, in part, by the gas turbine design geometry. The gas turbine fuel mass flow is identical to the combustion chamber fuel flow. The ratio of one single burner""s fuel flow to the combustion chamber fuel mass flow is, however, dependent on the number n and configuration of the active burners. Hence, the different equivalence ratios are closely related by
"PHgr"CC=ƒ("PHgr"GT), andxe2x80x83xe2x80x83equation 5 
"PHgr"SB=ƒ("PHgr"CC,n).xe2x80x83xe2x80x83equation 6 
In gas turbine combustion chambers with multiple burners, the burner technology is preferably geared to, but not limited to a lean combustion technology reducing emissions. The combustion chamber""s burners are switched on and off individually or are arranged in separately switchable burner groups. In order to achieve stable combustion and low emissions, the number of individual burners or burner groups in operation are varied over the range of operation.
Switching burners on or off with a constant "PHgr"GT distributes a certain amount of fuel to the burners leading to a shift in the burner equivalence ratio "PHgr"SB. If the number of active burners is reduced, the burner equivalence ratio "PHgr"SB will increase and consequently higher flame temperatures and higher emissions will occur. On the other hand, the activation of further burners will reduce the equivalence ratio "PHgr"SB. If the combustion process operates too closely to the extinction limit prior to the switch, some or even all burners will extinguish.
In conventional gas turbines load changes during start-up, shutdown or load following mode are accomplished by changing the air and/or fuel mass flow. In case of a load increase, relevant changes in the combustion chamber can occur such as:
The gas turbine inlet mix temperature TGT TIT and/or the air mass flow increases.
The combustion chamber heat load increases (given by the product of combustion chamber air mass flow and the temperature difference between combustion chamber air inlet temperature and combustion chamber exhaust outlet temperature).
The total fuel mass flow into the combustion chamber increases with the heat load.
The combustion chamber equivalence ratio "PHgr"CC increases with the temperature difference between combustion chamber air inlet temperature and combustion chamber exhaust outlet temperature.
Basically the operation of the combustion chamber can respond to an increased heat load in three ways. More fuel is added with either a reduced, same, or increased number of active burners.
Whenever the combustion chamber equivalence ratio "PHgr"CC increases, a constant or even reduced number of burners will result in a higher burner equivalence ratio "PHgr"SB. Hence, if the number of burners is not increased in order to compensate for the increased "PHgr"CC, emissions and single burner heat load will increase. But even if "PHgr"CC does not change, the higher heat load may allow for burner operation closer to the lean extinction limit. Hence, switching on additional single burners when increasing the gas turbine load may be advantageous to reduce emissions and heat load of the single burners. A reduced single burner heat load will, in turn, reduce the thermal stress of single burners.
FIG. 2 shows several gas turbine temperatures that are of interest concerning the operation of a gas turbine and its combustion chamber. Herein the temperature of the premixed flame can be found at position 10. The combustion gases are cooled down by air bypassing the main reaction zone and reentering the combustion chamber at position 20. The so-called xe2x80x9cfiring temperaturexe2x80x9d is defined as the temperature directly upstream of the turbine""s first vane row at position 30. This temperature and the temperature upstream of the first moving blade row at position 40 are limited with respect to the vane and blade material. To ensure their mechanical integrity both vanes and blades of modern gas turbines are usually cooled with air or steam. The temperature that would be found, if all the cooling media were mixed with the combustion chamber exhaust gases, is defined as the gas turbine inlet mix temperature TGT TIT. This temperature cannot be measured, however it can be determined by calculation.
Criteria are needed to define the burner or burner group switch points in a gas turbine control program. The burner switch must comply with low emission requirements without endangering combustion stability or risking sudden flame loss by extinction. Theoretically, the equivalence ratio is a suitable criterion to determine burner switches.
The equivalence ratio according to equation 1 is a normalized ratio of fuel and oxidizer mass flow. However, in order to use this ratio as a switching criterion, the fuel and oxidizer mass flows at the burners must be known.
In an open gas turbine cycle, the oxidizer mass flow that is discharged by the compressor cannot be measured. A theoretical prediction is possible, however, it is not reliable for the oxidizer mass flow through the burners for the following reasons
The compressor discharge mass flow and temperature change according to ambient conditions, where ambient temperature and pressure are most relevant,
the compressor discharge mass flow as well as the temperature can change with time due to compressor aging and fouling.
Therefore, a reliable prediction of the burner oxidizer flow is limited in practice, and the equivalence ratio is not a suitable criterion for determining the burner switch points. Other criteria describing the combustion in the burner or the combustion chamber, such as for example the temperature increase of single burners, gas turbine inlet temperature, are also based on the compressor mass flow and hence are also not suitable.
All criteria involving the chemical reaction require knowledge of the oxidizer mass flow through the burner. However, the criterion to determine burner switches must be measurable or calculable with high accuracy.
Such a criterion is the gas turbine load, which can be easily measured at the generator terminal. However, in the application of this criterion variations in the compressor discharge temperatures and/or mass flow must also be taken into consideration in the control of the combustion process.
It is the objective of this invention to provide a method of operating a gas turbine in which burner switch points are determined by a criterion that circumvents the disadvantages described in the state of the art.
The invention pertains to a method of operating a gas turbine arranged in a power generation system and comprising a source of compressed air, a combustor having a combustion chamber and multiple burners. In the method, a gas turbine controller controls the activation and deactivation of the burners according to a switch point criterion that is proportional to the difference between the temperature of compressed air upstream of the combustion chamber and the temperature of the exhaust downstream of the combustion chamber. The temperature of the compressed air upstream of the combustion chamber is preferably that immediately before the combustion chamber. In the case that the power generation system is a compressed air energy storage system, this temperature can also be the temperature at a point before the air turbine or even before the recuperator. The exhaust temperature downstream of the combustion chamber is the temperature of the exhaust at any point downstream of the point at which the chemical reaction has been completed.
In a particular method according to the invention the temperature downstream of the combustion chamber is the gas turbine inlet mix temperature, which is a virtual temperature calculated from a mixture of the exhaust gases and cooling media. These cooling media are admitted to one or more of the following, burners, combustion chamber, and gas turbine. The temperature difference for the switching criterion may then be expressed by KS, which signifies the difference between the gas turbine inlet mix temperature and the combustion chamber inlet temperature of the compressed air and is expressed by the following equation 7:
KS=(TGT TITxe2x88x92TCCinlet). 
This temperature KS, difference is physically related to various equivalence ratios of the combustion process. For example, it is proportional to the equivalence ratio "PHgr"CC in the combustion chamber, that is to the ratio of the fuel mass flow rate to the air mass flow rate through the combustion chamber, according to equation 8:       Φ    CC    ∝                    m        .            fuel                      m        .            air        ∝      (                  T                  GT          ⁢                      xe2x80x83                    ⁢          TIT                    -              T        CCinlet              )  
As well as to the equivalence ratio of the gas turbine, according to equation 9:       Φ    GT    ∝      Φ    CC    ∝                    m        .            fuel                      m        .            air        ∝      (                  T                  GT          ⁢                      xe2x80x83                    ⁢          TIT                    -              T        CCinlet              )  
Furthermore, the criterion is also related to the single burner equivalence ratio "PHgr"SB via the number n of active burners as expressed by equation 6.
To apply the proposed concept, the gas turbine inlet mix temperature TGT TIT, for example according to ISO 2314 standard, must be known. This temperature cannot be measured, but must be calculated, for example from a combustion chamber energy balance. For an accurate calculation, the air mass flow through the combustion chamber must be known.
If the air mass flow admitted to the gas turbine can be measured with high accuracy, the switching criterion according to the invention is advantageous. Therefore, application of the criterion is fully applicable in CAES power plants, as shown in FIG. 1, into which air is admitted from the cavern 1 and where an accurate measurement of air mass flow to the combustion chamber is possible. In a CAES power plant as outlined in FIG. 1, variations in the compressor discharge temperature are equivalent to changes in the air turbine exit temperature. These might occur if the air turbine inlet temperature has changed due to fluctuations in duct burner power.
An application of the switching criterion in conventional gas turbines is limited by the fact that the air mass flow from the compressor to the gas turbine is not reliably predictable.
The switching criterion according to the invention is applicable to the activation and deactivation of single burners as well as groups of individual burners.
In a particular variant of the method the burners are activated and deactivated in order to vary and control the heat load of the burners. This allows, for example, a limitation of the heat load in regard to the lean extinction limit or to permissible emission levels.
By the method of operating a gas turbine according to the invention, changes in temperature of the compressor discharge or of the compressed air from a storage cavern that has been led through a recuperator and air turbine is taken into direct account in the control of the individual burners or groups of burners. By this method those disadvantages are avoided that are typically encountered in power plants whose burners are controlled by a switching criterion based on the load. Specifically, as a result of the activation and deactivation of the burners according to the switching criterion disclosed here, the equivalence ratios of the combustion process vary within a smaller range for a given range of temperature variation of the compressed air admitted to the combustion chamber. The smaller range of values of the equivalence ratios allow a greater flexibility in the design of the combustion chamber and a greater range for the operating parameters. Furthermore, critical operating ranges in regard to emissions and lean extinction limit are less likely to be reached.